# Explain how the structure of a vein is adapted to facilitate the return of blood to the heart.

> IB DP Biology Higher Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks

## Mark scheme (4 marks)

1. Veins have a wide lumen, which reduces resistance to blood flow / allows a large volume of blood to move at low pressure.
2. Veins have thin walls / relatively little smooth muscle or elastic tissue, because blood pressure is low / they do not need to withstand high pressure.
3. Veins contain valves (pocket/semilunar valves) that prevent the backflow of blood / ensure one-directional flow towards the heart.
4. Surrounding skeletal muscle contractions compress veins / exert pressure on vein walls, pushing blood towards the heart; valves ensure this movement is unidirectional.

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structure-of-a-c204d0d9) · Published by Druglandscape Ltd.